PM / AVS: SmartReflex: use omap_sr * for enable/disable interface

SmartReflex driver interface is natively divided to two parts:

- external SmartReflex interface
- interface between SmartReflex driver and SmartReflex Class

Functions which belong to AVS class interface can use
struct omap_sr* instead of struct voltatedomain*, to provide a
direct connection between SR driver and SR class. This allows
us to optimize and not do additional lookups where none is
required.

sr_enable() and sr_disable() are interface functions between
SR driver and SR class. They are typically used by Class driver
to enable/disable SmartReflex hardware module.
Now they take struct omap_sr* as input parameter.

Signed-off-by: Andrii Tseglytskyi <andrii.tseglytskyi@ti.com>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Kevin Hilman <khilman@linaro.org>